Systems and methods for communicating high speed signals in a communication device

ABSTRACT

A coupling module can be used to communicate high speed signals between an optical transceiver and a processing module of an optical communication device, such as an optical line termination (OLT) or an optical network unit (ONU). The coupling module can adjust the common mode voltage level of a differential signal output by the optical transceiver to the common mode voltage level required by the processing module. In addition, the coupling module splits each of the differential output signals from the optical transceiver and passes the split signals to both a high-pass filter and a low-pass filter that are connected in parallel. The outputs of the high-pass filter and the low-pass filter from different paths of the differential signal are cross-coupled and combined to provide a differential signal to the processing module.

BACKGROUND

The present application generally relates to systems and methods forcommunicating high speed signals between a transceiver and a processingmodule in a communication device, such as an optical line termination.

In an optical line termination, an optical transceiver receives anoptical signal modulated with a data stream from an optical network unitand converts the optical signal to an electrical signal. The electricalsignal from the optical transceiver is provided to a processing modulefor further processing as may be desired. Frequently, the opticaltransceiver and the processing module are provided in separate circuits(possibly in separate cards or separate integrated circuits) havingdifferent common mode voltage level or DC (direct current) offsetvoltage level requirements. For example, the optical transceiver canoutput a signal having a common mode voltage (e.g., 3.3 V) that issignificantly greater than the common mode voltage (e.g., 1.2 V or 0.6V) for the signal that can be received by the processing module. Thus,the electrical signal from the optical transceiver cannot be provideddirectly to the processing module because the processing module is notequipped to handle the signal with the higher common mode voltage.

For compatibility between the optical transceiver and the processingmodule, the common mode voltage for the signal from the opticaltransceiver should be level shifted so that the signal can be receivedby the processing module. One way to couple the optical transceiver tothe processing module to obtain the desired level shift is withcapacitive coupling. One drawback to capacitive coupling is that thecoupling capacitors do not provide an appropriate DC response and maynot function properly if burst mode operation is required in the opticalline termination. Another way to couple the optical transceiver to theprocessing module to obtain the desired level shift is with bustransceivers. A drawback to the use of bus transceivers is that theyundesirably introduce jitter into the signal. Both of the previouslydescribed ways of coupling the optical transceiver to the processingmodule also have a drawback in that they are designed for use with asingle type of optical transceiver and would not provide an appropriatecommon mode voltage to the processing module if coupled to a differenttype of optical transceiver.

SUMMARY

The present application generally pertains to a coupling module in acommunication device, such as an optical line termination (OLT) oroptical network unit (ONU), that communicates high speed signals, i.e.,signals transmitted at 1 Gbps (Gigabit per second) or greater, between atransceiver and a processing module. The coupling module can provide thecommon mode voltage level desired by the processing module substantiallyindependent of the common mode voltage level of the signal output by thetransceiver.

The coupling module can receive a differential signal from thetransceiver and split each of the positive signal and the negativesignal that form the differential signal for additional processing. Thesplit signals for both the positive differential signal path and thenegative differential signal path can then be provided to a high-passfilter and a low-pass filter that are connected in parallel. Forexample, the positive differential signal can be split into two signalswith one being provided to a high-pass filter and the other beingprovided to a low-pass filter. The low-pass filter can have an invertingconfiguration such that the output from the low-pass filter is invertedwith respect to the input. The outputs of the high-pass filters from thedifferential signal paths are cross-coupled to the outputs of thelow-pass filters of the other differential signal paths to correct forthe inversion of the signal from the low-pass filter. For example, theoutput of the high-pass filter connected in the positive differentialsignal path is connected to the output of the low-pass filter connectedin the negative differential signal path. The cross-coupled signals arethen combined to form a differential signal that is provided to theprocessing module.

The high-pass filter and the low-pass filter can be configured such thatone or more predetermined ranges of frequencies of the signal from thetransceiver are provided to the processing module without anysignificant phase shift. In addition, the low-pass filters can beconfigured to provide the common mode voltage required by the processingmodule for a range of input common mode voltages provided by thetransceiver. To accomplish the level shift of the common mode voltagefrom the transceiver, the common mode voltage outputs of the low-passfilters can be measured by a feedback circuit that then provides anappropriate input to the low-pass filters such that the common modevoltage output from the low-pass filters is at the appropriate level forthe processing module.

One advantage of the present application is the jitter-freecommunication of high speed signals between an optical transceiver and aprocessing module in an optical communication device.

Another advantage of the present application is that the coupling modulecan simultaneously provide DC coupling, signal integrity, and wide (GHzto multi-GHz) bandwidth while maintaining signal swing.

Other features and advantages of the present application will beapparent from the following more detailed description of the identifiedembodiments, taken in conjunction with the accompanying drawings whichshow, by way of example, the principles of the application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an embodiment of a passive opticalnetwork.

FIG. 2 is a block diagram showing various components of an embodiment ofan optical line termination.

FIG. 3 is a block diagram showing various components of an embodiment ofthe coupling module of FIG. 2.

FIG. 4 is a circuit diagram showing an embodiment of the coupling moduleof FIG. 3.

FIG. 5 is a circuit diagram showing another embodiment of the couplingmodule of FIG. 3.

FIG. 6 is a graph showing an exemplary frequency response from thecoupling module of FIG. 5.

FIG. 7 is a circuit diagram showing a further embodiment of the couplingmodule of FIG. 3.

FIG. 8 is a block diagram showing various components of anotherembodiment of an optical line termination.

FIG. 9 is a block diagram showing various components of embodiments ofthe adapter module and the coupling module of FIG. 8.

FIG. 10 is a circuit diagram showing an embodiment of the voltage moduleof FIG. 9.

FIG. 11 is a block diagram showing another embodiment of the couplingmodule of FIG. 2.

Wherever possible, the same reference numbers are used throughout thedrawings to refer to the same or like parts.

DETAILED DESCRIPTION

The present application generally pertains to a coupling moduleconnecting an optical transceiver and a processing module in an opticalcommunication device, such as an optical line termination (OLT) oroptical network unit (ONU). The coupling module can include a couplingnetwork that connects a driver circuit of the optical transceiverproviding a differential signal to a receiver circuit of the processingmodule receiving a differential signal. In one embodiment, the couplingnetwork can include a purely passive high-pass filter in parallel withan op-amp (operational amplifier) based low-pass filter for both thepositive path and the negative path of the differential signal. Thehigh-pass filters can include a capacitor to reduce jitter. The low-passfilter can incorporate an op-amp configured in an inverting mode andhave a resistor and a capacitor connected in parallel at the output ofthe op-amp. In addition, the low-pass filter can be used to control thecommon mode voltage of the differential signal provided to theprocessing module. A feedback circuit can be coupled to receive theoutputs of the op-amps of the low-pass filters and provide an inputvoltage to the op-amps so that the outputs of the op-amps have a commonmode voltage expected by the receiver of the processing module. Thefeedback circuit can include an op-amp that receives the output commonmode voltages from the op-amps of the low-pass filters and a referencevoltage that corresponds to the expected common mode voltage for thereceiver of the processing module as inputs. The output of the low-passfilter on one path of the differential signal can be cross-coupled tothe output of the high-pass filter on the other path of the differentialsignal with a linking circuit. In another embodiment, output of thehigh-pass filter on one path of the differential signal can becross-coupled to the output of the low-pass filter on the other path ofthe differential signal with a linking circuit. The cross-coupling ofthe outputs from the high-pass filters and the low-pass filters cancompensate for the operation of the op-amps of the low-pass filters inthe inverting mode.

The high-pass filter and the low-pass filter can be tuned to preservethe signals in a transition region between the two filters (i.e., aregion where the outputs of the filters overlap). However, in otherembodiments, there can be gap in the transition region between thehigh-pass filter and the low-pass filter. The gap in the transitionregion between the filters can be predefined by tuning both thehigh-pass filter and the low-pass filter such that the parameters (e.g.,width) of the gap are known. In a further embodiment, one or both of thehigh-pass filter and the low-pass filter may not be tuned and may resultin a gap in the transition region.

FIG. 1 depicts an embodiment of a passive optical network (PON) 39 forcommunicating data with customer premises equipment (CPE) 15. Examplesof PONs and telecommunication systems that can be used with the presentapplication are described in commonly-assigned U.S. Pat. No. 9,729,241,entitled “Telecommunication Systems and Methods Using Dynamic Shapingfor Allocating Network Bandwidth” and granted on Aug. 8, 2017, which isincorporated herein by reference.

As shown by FIG. 1, the PON 39 includes an optical line termination(OLT) 30. In one embodiment, the OLT 30 resides on a line card of anetwork access device (NAD) 22, which may include other OLTs of otherPONs, as is described by U.S. Pat. No. 9,729,241. The NAD 22 can be usedto facilitate communications, both upstream and downstream, between theCPEs 15 and a telecommunication network (not shown). As an example, thenetwork access device 22 may reside at a central office of atelecommunication network or an intermediate point between a centraloffice and the CPEs 15.

The OLT 30 can be coupled to an optical splitter 37 by an optical fiber34, and the optical splitter 37 is configured to split a signal from theOLT 30 across multiple optical fibers 35 that are respectively coupledto ONUs 33 as shown. Each ONU 33 can receive at least one packet flowfrom the OLT 30 and convert the received packet flow(s) from the opticaldomain to the electrical domain. The OLT 30 and the optical componentscoupled to it, including the optical splitter 37, ONUs 33, and opticalfibers 34, 35 form the PON 39. In one embodiment, the PON 39 is agigabit passive optical network (GPON), but other types of PONs arepossible in other embodiments.

FIG. 2 shows an embodiment of an OLT 30 with the components used forprocessing upstream communications from an ONU 33. It is to beunderstood that the OLT 30 shown in FIG. 2 may include additionalequipment and/or components to perform additional functions andoperations that are not shown in FIG. 2, e.g., the processing ofdownstream communications. The OLT 30 can have an optical transceiver 63that receives an upstream optical signal from an ONU 33 via opticalfiber 34. The signal carries a data stream transmitted by the ONU 33. Inone embodiment, the signal from the ONU 33 can be a high speed signalcarrying the data stream at a data rate of between about 1 Gbps andabout 10 Gbps or greater, though other data rates are possible. Theoptical transceiver 63 converts the received optical signal to anelectrical signal and provides the electrical signal to an inputconnection 64 of the coupling module 60 as a differential pair ofsignals. The coupling module 60 adjusts the level of the common modevoltage or DC offset voltage of the electrical signal and provides anelectrical signal with an adjusted common mode voltage as a differentialpair of signals to a processing module 69 via output connection 68. Notethat in one embodiment all of the components of the OLT 30 can reside ona printed circuit board (PCB), referred to as a “line card.” In otherembodiments, other configurations of the OLT 30 are possible.

The optical transceiver 63 can include a photo detector, such as anavalanche photo diode, to convert the optical signal to an electricalsignal. The optical transceiver 63 can also include an amplifier circuitsuch as a trans-impedance amplifier and a driver circuit to provide theelectrical signal to the input connection 64 of the coupling module 60.In addition, the optical transceiver 63 can be configured to be eitherDC coupled or AC (alternating current) coupled. The AC-coupled opticaltransceiver 63 includes a capacitor connected between the driver circuitand an output connection that is coupled to the input connection 64 ofthe coupling module 60. The common mode voltage of the electrical signalfrom the optical transceiver 63 can range between about 2.5 V and about3.3 V, although other voltage ranges are possible in other embodiments.

The processing module 69 can include a receiver circuit to receive thesignal from an output connection 68 of the coupling module 60. Theprocessing module 69 can also include a field programmable gate array(FPGA) and/or other electrical components to further process thereceived signal. In one embodiment, the common mode voltage of theelectrical signal provided on the output connection 68 from the couplingmodule 60 can be less than about 1.3 V to correspond to the desiredcommon mode voltage of the processing module 69. In another embodiment,the required common mode voltage of the processing module 69 can beabout 0.6 V, though other common mode voltages are possible. Thecoupling module 60 can shift or adjust the level of the common modevoltage of the differential pair of signals provided at input connection64 such that the common mode voltage level of the differential pair ofsignals provided at the output connection 68 is acceptable for theprocessing module 69.

As shown in FIGS. 3 and 4, the coupling module 60 can receive from thetransceiver 63 a positive differential signal at a positive ornon-inverting path (+ path) and a negative differential signal at anegative, inverting or complementary path (− path) of the inputconnection 64. Similarly, the coupling module 60 can provide adifferential signal (e.g., a positive differential signal and a negativedifferential signal that is inverted with respect to the positivedifferential signal) for the processing module 69 at a positive ornon-inverting path (+ path) and a negative, inverting or complementarypath (− path) of the output connection 68. The coupling module 60 canadjust the level of the common mode voltage of the input differentialsignal at the input connection 64 such that the output differentialsignal provided by the coupling module 60 at the output connection 68corresponds to the common mode voltage expected by the processing module69.

The coupling module 60 can provide the input positive differentialsignal on the positive path (+ path) to both a high-pass filter 65P anda low-pass filter 67P connected in parallel. Similarly, the couplingmodule 60 can provide the input negative differential signal on thenegative path (− path) to both a high-pass filter 65N and a low-passfilter 67N connected in parallel. The input negative differential signalcan be an inversion of the input positive differential signal in oneembodiment. The coupling module 60 can include a linking circuit 70P tocombine the outputs of the high-pass filter 65P and the low-pass filter67N to form the output positive differential signal provided on thepositive path (+ path) of output connection 68 and a linking circuit 70Nto combine the outputs of the high-pass filter 65N and the low-passfilter 67P to form the output negative differential signal provided onthe negative path (− path) of output connection 68. The output negativedifferential signal can be an inversion of the output positivedifferential signal in one embodiment.

The high-pass filters 65P, 65N filter the low frequency signals from theinput differential signals and permit the high frequency signals above apredetermined frequency from the input differential signals to pass tothe corresponding linking circuits 70P, 70N and then the outputconnection 68. The low-pass filters 67P, 67N filter the high frequencysignals from the input differential signals and permit the low frequencysignals below a predetermined frequency to pass to cross-coupled linkingcircuits 70P, 70N and then the output connection 68. In addition, thelow-pass filters 67P, 67N also shift or adjust the level of the commonmode voltage of the input differential signals based on feedback fromfeedback circuit 72 such that the common mode voltage level of theoutput differential signals at the output connection 68 is acceptablefor the processing module 69.

As shown in FIG. 4, the high-pass filters 65P, 65N can each include acapacitor 82 to filter out the low frequency signals in the inputdifferential signals from the optical transceiver 63 and permit the highfrequency signals above a predetermined frequency in the inputdifferential signals from the optical transceiver 63 to pass to thecorresponding linking circuits 70P, 70N and then the output connection68. In one embodiment, the capacitor 82 can have a capacitance of about0.1 μF. In another embodiment, the capacitor 82 can have a capacitanceof about 0.01 μF. However, capacitor 82 may use different capacitancesin still other embodiments.

The low-pass filters 67P, 67N include operational amplifiers (op-amps)90P, 90N operated in the inverting mode (i.e., the input signal isconnected to the inverting node of the op-amp). The op-amps 90P, 90N areused to control the level of the common mode voltage of the outputdifferential signal provided at the output connection 68 since thecapacitors 82 of the high-pass filters 65P, 65N block the common modevoltage received from the optical transceiver 63. In one embodiment, theop-amps 90P, 90N can include a Texas Instruments OPA2830 providingsufficient bandwidth and a low noise figure. However, other op-ampshaving different bandwidths and noise figures may be used in otherembodiments.

The input positive differential signal from the positive path (+ path)passes through resistor R1P and is received at the inverting input ofop-amp 90P. Similarly, the input negative differential signal from thenegative path (− path) passes through resistor R1N and is received atthe inverting input of op-amp 90N. The non-inverting inputs of theop-amps 90P, 90N can be biased at a fixed voltage within the common moderange of the op-amps 90P, 90N as described below. In addition, feedbacksignals from the outputs of the op-amps 90P, 90N can pass throughcorresponding resistors R2P, R2N and be provided at the inverting inputsof the op-amps 90P, 90N. In one embodiment, a predetermined ratio forR2P/R1P and R2N/R1N can be used to generate a flat frequency responsefrom the op-amps 90P, 90N. In an embodiment, resistors R1P and R1N caneach have a resistance of about 5 kΩ and resistors R2P and R2N can eachhave a resistance of about 23 kΩ. However, in other embodiments, otherresistances may be used for resistors R1P, R1N, R2P and R2N.

The low-pass filters 67P, 67N can also include capacitors C1P, C1N andresistors R3P, R3N connected to the output of the op-amps 90P, 90N. Thecorresponding capacitors C1P, C1N and resistors R3P, R3N can beconfigured to filter out the high frequency signals in the differentialsignals received at the input connection 64 and permit the low frequencysignals in the differential signals received at the input connection 64to pass to the output connection 68. The resistors R3P, R3N can beconnected in series with the corresponding outputs of the op-amps 90P,90N and the capacitors C1P, C1N can be connected in parallel with theresistors R3P, R3N. The capacitors C1P, C1N and resistors R3P, R3N canbe configured to provide a predetermined low frequency response suchthat when combined with the predetermined high frequency response of thehigh-pass filters 65P, 65N, the predetermined high frequency responseand the predetermined low frequency response are tuned to provide theoutput connection 68 with all the frequencies of the differentialsignals received at the input connection 64 without any substantialphase shift in the signals. In one embodiment, the capacitors C1P, C1Ncan have a capacitance of about 0.1 μF and the resistors R3P, R3N canhave a resistance between about 28Ω and about 40Ω, but the capacitorsC1P, C1N and resistors R3P, R3N may have different values in otherembodiments.

Since the input differential signals are provided to the inverting inputof the op-amps 90P, 90N (i.e., the op-amps 90P, 90N are operated in aninverting configuration), the output signals from the op-amps 90P, 90Nare inverted with respect to the input differential signals. In order tocompensate for the inversion of the signal by the op-amps 90P, 90N, theoutputs of the op-amps 90P, 90N are cross-coupled to the outputs ofhigh-pass filters 65P, 65N such that positive differential signals fromthe high-pass filter 65P are combined with positive differential signalsfrom op-amp 90N (which inverted the negative differential signalreceived at the inverting input to op-amp 90N) and negative differentialsignals from the high-pass filter 65N are combined with negativedifferential signals from op-amp 90P (which inverted the positivedifferential signal received at the inverting input to op-amp 90P).

FIG. 4 also shows the linking circuits 70P, 70N connecting the output ofthe high-pass filter 65P and the output of the low-pass filter 67N tothe positive path (+ path) of the output connection 68 and the output ofthe high-pass filter 65N and the output of the low-pass filter 67P tothe negative path (− path) of the output connection 68. The linkingcircuits 70P, 70N can be used to establish the gain of the high passpaths through the high-pass filters 65P, 65N. The gain of thecross-coupled low pass paths through the low-pass filters 67N, 67P canbe matched to the gain of the high pass paths through the high-passfilters 65P, 65N using the ratio of resistors R1P, R1N and resistorsR2P, R2N. In one embodiment, the resistors R1P, R1N and resistors R2P,R2N can be selected to provide a 50Ω impedance from either the high-passfilters 65P, 65N or the input connection 64. In another embodiment, thelinking circuits 70P, 70N can include resistors R4P, R4N connected inseries with the output of the high-pass filters 65P, 65N, resistors R5P,R5N connected in series between resistors R4P, R4N and the positive andnegative paths of the output connection 68, and resistors R6P, R6Nconnected to the output of the low-pass filters 67N, 67P and in parallelbetween resistors R4P, R4N and resistors R3N, R3P. In one embodiment,resistors R4P, R4N and resistors RSP, R5N can each have a resistancebetween about 3Ω and about 9Ω and resistors R6P, R6N can have aresistance between about 140Ω and about 433Ω. However, other resistancesmay be used for resistors R4P, R4N, RSP, R5N, R6P and R6N in otherembodiments. In another embodiment, the linking circuits 70P, 70N canalso operate as a 50Ω constant impedance, 3 dB attenuator to attenuateany reflections that may occur between the optical transceiver 63 andthe processing module 69. While the linking circuits 70P, 70N have beenshown in a “T” configuration in FIG. 4, the linking circuits 70P, 70Nmay have other configurations in other embodiments.

A feedback circuit 72 can sense the output common mode voltage from theop-amps 90P, 90N, compare the sensed output common mode voltage to areference voltage corresponding to the desired common mode voltage forthe processing module 69, and provide a signal back to the op-amps 90P,90N to adjust the common mode voltage at the output of the op-amps 90P,90N. The common mode voltage outputs from the op-amps 90P, 90N can besummed and provided to a feedback circuit 72 that can provide an inputto non-inverting inputs of the op-amps 90P, 90N such that the commonmode voltage provided at the output of the op-amps 90P, 90N is at anacceptable level for the processing module 69. The feedback circuit 72permits the op-amps 90P, 90N to provide the acceptable common modevoltage for the processing module 69 for a wide range of input commonmode voltages received from the transceiver 63 at the input connection64.

The feedback circuit 72 can include a resistor R7 connected to theoutput of op-amp 90P and a resistor R8 connected to the output of op-amp90N. The resistors R7 and R8 are also connected to the inverting inputof an op-amp 90F. The non-inverting input of the op-amp 90F can beconnected to a voltage source 104 that can provide an input voltage thatcorresponds to the acceptable voltage for the processing module 69. Inanother embodiment, the voltage source 104 can be replaced with aconnection to the processing module 69 that permits the processingmodule 69 to provide an input voltage to the non-inverting input ofop-amp 90F that is substantially equal to the acceptable common modevoltage of the processing module 69. A capacitor C2 can be connectedbetween the inverting input to the op-amp 90F and the output of theop-amp 90F. In one embodiment, the capacitor C2 can have a capacitanceof about 100 pF and the resistors R7 and R8 can have a resistance ofabout 10 kΩ, but the capacitor C2 and resistors R7 and R8 may havedifferent values in other embodiments. In addition, the output of theop-amp 90F is provided to the non-inverting inputs of the op-amps 90P,90N and can be used to automatically set the common mode output voltagefrom the op-amps 90P, 90N.

In one embodiment, the feedback circuit 72 (i.e., resistors R7 and R8,the op-amp 90F and the capacitor C2) can be configured as an integratorhaving an input that is the difference between the reference voltagefrom the voltage source 104 (which corresponds to the desired oracceptable common mode voltage for the processing module 69) and thecommon mode voltage output voltage output from op-amps 90P, 90N. Theop-amp 90F can then provide a voltage at the non-inverting inputs ofop-amps 90P, 90N that results in a common mode voltage output from theop-amps 90P, 90N that is equal to the reference voltage. The outputvoltage from the op-amp 90F can be compatible with the operation of theop-amps 90P, 90N. In other words, the output voltage level from theop-amp 90F is not close to the supply voltage level for the op-amps 90P,90N. In one embodiment, the output voltage from the op-amp 90F can rangebetween about 224 mV and about 2.29 V. The feedback circuit 72 canprovide the appropriate voltages to the non-inverting inputs of theop-amps 90P, 90N to generate the desired common mode voltage output fromthe op-amps 90P, 90N for substantially all common mode voltages (e.g., 0to 3.3 V) and impedances (e.g., between 0 (if an inductor is present) toinfinity (if a capacitor is present) at DC) at the optical transceiver63.

The op-amps 90P, 90N, 90F can receive “dual-rail” supply voltages of Vccand Vee.

In one embodiment, Vcc can be about 3.3 V and Vee can be about −3.3 Vand the coupling module 60 shown in FIG. 4 can have about 3 dB ofinsertion loss. In another embodiment, the supply voltages can beincreased and Vcc can be about 5 V and Vee can be about −5 V and thecoupling module 60 shown in FIG. 4 can have about 1 dB of insertionloss.

FIG. 5 shows an embodiment of a coupling module 60 similar to theembodiment of the coupling module 60 shown in FIG. 4. However, thecoupling module 60 of FIG. 5 also includes low-pass filter circuits 50P,50N connected between the positive and negative paths of the inputconnection 64 and the resistors R1P, R1N of the low-pass filters 67P,67N. The low-pass filter circuits 50P, 50N can be used to prevent theop-amps 90P, 90N of the low-pass filters 67P, 67N from receiving (andhaving to process) higher frequency signals, which higher frequencysignals may affect the operating points of the op-amps 90P, 90N. In oneembodiment, the low-pass filter circuits 50P, 50N can be configured toprovide an input pole below about 1 MHz, though other frequencies arepossible. The low-pass filter circuits 50P, 50N can include resistorsR9P, R9N connected in series with resistors R1P, R1N, and capacitorsC3P, C3N connected in parallel between resistors R9P, R9N and resistorsR1P, R1N. In one embodiment, the capacitors C3P, C3N can have acapacitance of about 100 pF and the resistors R9P, R9N can have aresistance between about 2 kΩ, but the capacitors C3P, C3N and resistorsR9P, R9N may have different values in other embodiments. FIG. 6 shows anembodiment of the frequency response from the coupling module 60 of FIG.5 with the solid curve showing the amplitude response and the dashedcurve showing the phase response. As shown in FIG. 6, the disturbance inthe frequency response is less than about 0.22 dB total.

FIG. 7 shows an embodiment of a coupling module 60 similar to theembodiment of the coupling module 60 shown in FIG. 5. However, thecoupling module 60 of FIG. 7 has a different feedback circuit 72 toprovide inputs to the low-pass filter circuits 67P, 67N. The feedbackcircuit 72 can include a resistor R7 connected to the output of op-amp90P and a resistor R8 connected to the output of op-amp 90N. Theresistors R7 and R8 are also connected to the inverting input of anop-amp 90F. The non-inverting input of the op-amp 90F can be connectedto a voltage source 104 that can provide an input voltage thatcorresponds to the acceptable voltage for the processing module 69. Inanother embodiment, the voltage source 104 can be replaced with aconnection to the processing module 69 that permits the processingmodule 69 to provide an input voltage to the non-inverting input ofop-amp 90F that is substantially equal to the acceptable common modevoltage of the processing module 69. A resistor R10 can be connectedbetween the inverting input to the op-amp 90F and the output of theop-amp 90F. In one embodiment, the resistor R10 can have a resistance ofabout 221 kΩ and the resistors R7 and R8 can have a resistance of about5.6 kΩ, but the resistors R7, R8 and R10 may have different values inother embodiments.

In addition, the output of the op-amp 90F is provided to thenon-inverting inputs of the op-amps 90P, 90N after being provided to astabilization circuit 98 to prevent oscillations from the outputs ofop-amps 90P, 90N. The output of the op-amp 90F can be used toautomatically set the common mode output voltage from the op-amps 90P,90N. In one embodiment, the stabilization circuit 98 can include aresistor R11 connected in series between the output of the op-amp 90Fand the non-inverting inputs of the op-amps 90P, 90N. A capacitor C4 canbe connected between resistor R11 and ground. In one embodiment, thecapacitor C4 can have a capacitance of about 100 nF and the resistor R11can have a resistance of about 22.1 kΩ, but the capacitor C4 andresistor R11 may have different values in other embodiments.

FIG. 11 shows another embodiment of the coupling module 60 of FIG. 2. Inthe embodiment of FIG. 11, the low-pass filters 67P, 67N of the couplingmodule 60 can include operational amplifiers (op-amps) operated in thenon-inverting mode (i.e., the input signal is connected to thenon-inverting node of the op-amp). In addition, the output of thelow-pass filter 67P of the coupling module 60 of FIG. 11 can be coupledto linking circuit 70P and the output of the low-pass filter 67N of thecoupling module 60 of FIG. 11 can be coupled to linking circuit 70N. Thenon-inverting op-amps of the low-pass filters 67P, 67N can be used tocontrol the level of the common mode voltage of the output differentialsignal provided at the output connection 68. Examples of low-passfilters with op-amps operating in the non-inverting mode that can beused with the present application are described in commonly-assignedU.S. Pat. No. 9,891,638, entitled “Systems and Methods for CommunicatingHigh Speed Signals in a Communication Device” and granted on Feb. 13,2018, which is incorporated herein by reference.

The coupling module 60 can use the feedback circuit 72 to provide aninput signal (or voltage) to the low pass filters 67P, 67N. The feedbackcircuit 72 can include a differential feedback circuit 222 to receiveand process the output common mode voltage from the low pass filters67P, 67N sensed by sensing circuit 224, compare the sensed output commonmode voltage to a reference voltage corresponding to the desired commonmode voltage for the processing module 69, and provide a signal back tothe low pass filters 67P, 67N to adjust the common mode voltage at theoutput of the low pass filters 67P, 67N. In one embodiment, the sensingcircuit 224 can sense the common mode voltage output from the low passfilters 67P, 67N with a summing network that adds the common modevoltage provided by each of the low pass filters 67P, 67N. As shown inFIG. 11, the output of the differential feedback circuit 222 can besplit in order to be provided to both of the low pass modules 67P, 67N.

FIG. 8 shows an embodiment of an optical line termination 30 similar tothe embodiment of the optical line termination 30 shown in FIG. 2.However, the optical line termination 30 of FIG. 8 includes an adaptermodule 200 to permit the coupling module 60 to work with different typesand/or configurations of optical transceivers 63. In one embodiment, thecoupling module 60 can have any suitable configuration that adjusts thelevel of the common mode voltage or DC offset voltage of the electricalsignal received via input connection 64 from the adapter module 200 andprovides an electrical signal with an adjusted common mode voltage as adifferential pair of signals to a processing module 69 via outputconnection 68. In an embodiment, the optical transceiver 63 can have aCML (current-mode logic) driver configuration or an LVPECL (low voltagepositive emitter coupled logic) or LVPCL driver configuration each ofwhich has different output requirements. For example, the LVPECL driverhas an emitter/follower configuration with different currentrequirements than the CML driver, which does not have anemitter/follower. The adapter module 200 is configured to provideappropriate voltage levels and current paths for the different outputsfrom the optical transceiver 63 such that the coupling module 60receives an appropriate input at input connection 64.

As shown in FIG. 9, the adapter module 200 can receive a differentialsignal (e.g., a positive path and a negative path) from the opticaltransceiver 63 and provide a corresponding differential signal to theinput connection 64 for the coupling module 60. The coupling module 60can be configured as described above with regard to FIG. 4, 5 or 7. Thedifferential signal provided to the coupling module 60 at inputconnection 64 by the adapter module 200 can be a “spilt” differentialsignal. The split differential signal can include two separate positivepaths (i.e., one positive path for the high pass filter 65P and onepositive path for the low pass filter 67P) and two separate negativepaths (i.e., one negative path for the high pass filter 65N and onenegative path for the low pass filter 67N). Specifically, the signalsprovided by the adapter module 200 to the high pass filters 65P, 65N isthe differential signal from the optical module 63. The signals providedby the adapter module 200 to the low pass filters 67P, 67N is thedifferential signal from the optical module 63 after passing through aportion of a resistor divider 220. The resistor divider 220 can becoupled between the differential signal from the optical transceiver 63and a voltage adjustment circuit 210 that establishes a predeterminedvoltage (e.g., ground or a preselected voltage greater than 0 V) at theend of the resistor divider 220 opposite the differential signal fromthe optical transceiver 63.

The resistor divider 220 can include a pair of series-connectedresistors R221, R223 connected to the negative path of the differentialsignal from the optical transceiver 63 and a pair of series-connectedresistors R222, R224 connected to the positive path of the differentialsignal from the optical transceiver 63. In one embodiment, the resistorsR221, R222, R223 and R224 can each have a resistance of about 100Ω, butthe resistors R221, R222, R223 and R224 may have different values(either individually or as a group) in other embodiments. The two pairsof resistors can then be coupled together and connected to the voltageadjustment circuit 210. The corresponding portion of the differentialsignal provided to the low pass filters 67P, 67N can be provided from aconnection between resistors R221 and R223 (for the negative path) and aconnection between resistors R222 and R224 (for the positive path). Theportion of the differential signal provided to the low pas filters 67P,67N can include information associated with the DC level of thedifferential signal from the optical transceiver 63.

FIG. 10 is a circuit diagram showing an embodiment of the voltageadjustment circuit 210 to control the voltage level provided at the endof the resistor divider 220 opposite the connection to the differentialsignal from the optical transceiver 63. The voltage adjustment circuit210 can include a transistor T1 controlled by an op-amp 212 that setsthe voltage level that is pulled to by the resistor divider 220. Theemitter of transistor T1 can be connected to the resistor divider 220,the collector of transistor T1 can be connected to resistor R216 and thebase of transistor T1 can be connected to the output of op-amp 212. Inone embodiment, the transistor T1 can be a BJT (bipolar junctiontransistor), but the transistor T1 may be any suitable type oftransistor in other embodiments. A capacitor C213 and the non-invertinginput to the op-amp 212 can be connected to the resistor divider 220 inparallel with the emitter of transistor T1. A resistor R215 can beconnected in series with the output of the op-amp 212 and a capacitorcan be connected in parallel with resistor R215. In one embodiment,capacitor C212 can have a capacitance of about 27 pF, capacitor C213 canhave a capacitance of about 100 nF, the resistor R215 can have aresistance of about 1 kΩ and the resistor R216 can have a resistance ofabout 10Ω, but capacitors C212 and C213 and resistors R215 and R216 mayhave different values in other embodiments.

The non-inverting input to the op-amp 212 can be connected to a resistornetwork receiving voltage inputs (or bits) from a processing module 214.The processing module 214 can include a field programmable gate array(FPGA) and/or other electrical components to provide the correspondingvoltage inputs to the resistor network. The resistor network can providean input voltage to the non-inverting input of the op-amp 212 based onthe voltage inputs provided by the processing module 214. The processingmodule 214 can provide voltage inputs to the resistor network based onthe type of optical receiver 63 being used. The resistor network caninclude a capacitor C211 connected between the non-inverting input andground, a first pair of series-connected resistors R211 and R212connected in parallel with capacitor C211, and a second pair ofseries-connected resistors R213 and R214 connected in parallel withcapacitor C211 and with the first pair of series-connected resistorsR211 and R212. Resistors R212, R213 and R214 can be connected todifferent terminals of the processing module 214 to receive differentvoltage inputs from the processing module 214. In one embodiment, thecapacitor C211 can have a capacitance of about 1 nF, the resistor R211can have a resistance of about 10 kΩ, the resistor R212 can have aresistance of about 13 kΩ, the resistor R213 can have a resistance ofabout 100 MΩ, and the resistor R214 can have a resistance of about 49.9MΩ, but the capacitor C211 and resistors R211, R212, R213 and R214 mayhave different values in other embodiments.

In an embodiment, the output of the op-amp 212 can be used to controlthe transistor T1 such that the current from the resistor divider 220flows to capacitor 213 or flows through transistor T1 to resistor R216and ground. In another embodiment, the voltage adjustment circuit 210can be replaced by a fixed voltage source that provides a fixed voltageto the end of the resistor divider 220 opposite the connection to thedifferential signal from the optical transceiver 63. The fixed voltagefrom the fixed voltage source can be selected to permit the couplingmodule 60 to operate with several different types of optical modules 63.In one embodiment, the voltage for the fixed voltage source can beeither about 2.2 V or about 2.3. V.

Although the figures herein may show a specific order of method steps,the order of the steps may differ from what is depicted. Also, two ormore steps may be performed concurrently or with partial concurrence.Variations in step performance can depend on the software and hardwaresystems chosen and on designer choice. All such variations are withinthe scope of the application. Software implementations could beaccomplished with standard programming techniques, with rule based logicand other logic to accomplish the various connection steps, processingsteps, comparison steps and decision steps.

The coupling module 60 is described in various embodiments for usewithin an OLT. However, it is possible to use the coupling module 60 inother types of communication devices, such as an ONU. As an example, anONU may be configured according to the block diagram shown by FIG. 2having a coupling module 60 that is coupled between and opticaltransceiver 63 and a processing module 69, as described above for theOLT 30.

Further, the use of the coupling module 60 is not limited tocommunication devices. In other embodiments, the coupling module 60 canbe connected between an input module and an output module that require alevel shift of the common mode voltage or the DC offset voltage in orderfor the modules to communicate. The coupling module 60 can adjust thecommon mode voltage or DC offset voltage of a signal received from theinput module to enable the output module to process the signal.

It should be understood that the identified embodiments are offered byway of example only. Other substitutions, modifications, changes andomissions may be made in the design, operating conditions andarrangement of the embodiments without departing from the scope of thepresent application. Accordingly, the present application is not limitedto a particular embodiment, but extends to various modifications thatnevertheless fall within the scope of the application. It should also beunderstood that the phraseology and terminology employed herein is forthe purpose of description only and should not be regarded as limiting.

1. A coupling module comprising: an input connection having a first path and a second path, the first path configured to receive a first differential signal of an input signal and the second path configured to receive a second differential signal of the input signal; a first high-pass filter connected to the first path and configured to filter the first differential signal to provide a first filtered signal; a first low-pass filter connected to the first path and configured to filter the first differential signal to provide a second filtered signal; a second-high pass filter connected to the second path and configured to filter the second differential signal to provide a third filtered signal; a second-low pass filter connected to the second path and configured to filter the second differential signal to provide a fourth filtered signal; and an output connection having a third path and a fourth path, the third path configured to provide a third differential signal of an output signal and the fourth path configured to provide a fourth differential signal of the output signal, wherein the third path is connected to the first high-pass filter and the second low-pass filter for combining the first filtered signal with the fourth filtered signal to form the third differential signal, and wherein the fourth path is connected to the first low-pass filter and the second high-pass filter for combining the second filtered signal with the third filtered signal, and wherein the output signal has a common mode voltage acceptable to an output module coupled to the output connection.
 2. A coupling module comprising: an input connection configured to be connected to an input module to receive an input signal from the input module, the input connection having a first positive path configured to receive a first positive differential signal of the input signal and a first negative path configured to receive a first negative differential signal of the input signal; a first high-pass filter connected to the first positive path to receive the first positive differential signal, the first high-pass filter configured to filter the first positive differential signal and output a first filtered signal including signal frequencies greater than a first predetermined frequency; a second high-pass filter connected to the first negative path to receive the first negative differential signal, the second high-pass filter configured to filter the first negative differential signal and output a second filtered signal including signal frequencies greater than the first predetermined frequency; a first low-pass filter connected to the first positive path in parallel with the first high-pass filter to receive the first positive differential signal, the first low-pass filter configured to filter the first positive differential signal and output a third filtered signal including signal frequencies less than a second predetermined frequency, the first low-pass filter configured to provide the third filtered signal with a first common mode voltage; a second low-pass filter connected to the first negative path in parallel with the second high-pass filter to receive the first negative differential signal, the second low-pass filter configured to filter the first negative differential signal and output a fourth filtered signal including signal frequencies less than the second predetermined frequency, the second low-pass filter configured to provide the fourth filtered signal with a second common mode voltage; an output connection configured to be connected to an output module to provide an output signal to the output module, the output signal including a second positive differential signal and a second negative differential signal, the output connection having a second positive path configured to provide the second positive differential signal to the output module and a second negative path configured to provide the second negative differential signal to the output module, the second positive path coupled to the first high-pass filter and the second low-pass filter, the second positive differential signal being formed from a combination of the first filtered signal and the fourth filtered signal, the second negative path coupled to the second high-pass filter and the first low-pass filter, the second negative differential signal being formed from a combination of the second filtered signal and the third filtered signal; and wherein the first and second common mode voltages of the third and fourth filtered signals provide a common mode voltage in the output signal acceptable to the output module.
 3. The coupling module of claim 2, further comprising a feedback circuit connected to the first low-pass filter and the second low-pass filter, the feedback circuit configured to receive the third filtered signal and the fourth filtered signal and provide an input to the first low-pass filter and the second low-pass filter such that the first low-pass filter and the second low-pass filter adjust the first common mode voltage and the second common mode voltage to result in the common mode voltage acceptable to the output module.
 4. The coupling module of claim 3, wherein the feedback circuit comprises an integrator having an input corresponding to a difference between a reference voltage and a sum of the first common mode voltage and the second common mode voltage and an output corresponding to a voltage that results in the first low-pass filter adjusting the first common mode voltage in the third filtered signal and the second low-pass filter adjusting the second common mode voltage in the fourth filtered signal.
 5. The coupling module of claim 3, wherein the feedback circuit comprises an operational amplifier having a non-inverting input configured to receive a voltage and an inverting input configured to receive the third filtered signal and the fourth filtered signal.
 6. The coupling module of claim 5, wherein the feedback circuit further comprises a capacitor connected between the inverting input of the operational amplifier and an output of the operational amplifier, a first resistor connected between the inverting input of the operational amplifier and the first low-pass filter and a second resistor connected between the inverting input of the operational amplifier and the second low-pass filter.
 7. The coupling module of claim 2, wherein: the first high-pass filter and the second high-pass filter each comprise a capacitor having a capacitance, and wherein the capacitance of the capacitor is used to establish the first predetermined frequency; and the first low-pass filter comprises a first operational amplifier connected in series with the first positive path, the first operational amplifier configured to operate in an inverting mode such that the third filtered signal is inverted with respect to the first positive differential signal; and the second low-pass filter comprises a second operational amplifier connected in series with the first negative path, the second operational amplifier configured to operate in an inverting mode such that the fourth filtered signal is inverted with respect to the first negative differential signal.
 8. The coupling module of claim 7, wherein: the first low-pass filter further comprises: a first output circuit connected between an output of the first operational amplifier and the second negative path of the output connection; a first input circuit connected between an inverting input of the first operational amplifier and the first positive path of the input connection; and wherein at least one of the first input circuit or the first output circuit is used to establish the second predetermined frequency in the first low-pass filter; and the second low-pass filter further comprises: a second output circuit connected between an output of the second operational amplifier and the second positive path of the output connection; a second input circuit connected between an inverting input of the second operational amplifier and the first negative path of the input connection; and wherein at least one of the second input circuit or the second output circuit is used to establish the second predetermined frequency in the second low-pass filter.
 9. The coupling module of claim 8, wherein the first output circuit and the second output circuit each comprise a resistor connected in parallel with a capacitor.
 10. The coupling module of claim 8, wherein the first input circuit and the second input circuit each comprise a resistor.
 11. The coupling module of claim 10, wherein the first input circuit and the second input circuit each comprise a low-pass filter circuit connected to the resistor.
 12. The coupling module of claim 7, wherein the first operational amplifier and the second operational amplifier each receive a pair of supply voltages, wherein the pair of supply voltages is one of +3.3 volts and −3.3 volts or +5 volts and −5 volts.
 13. The coupling module of claim 2, wherein: the second positive path comprises a first linking circuit connecting the first high-pass filter and the second low-pass filter and configured to match gains of the first filtered signal and the fourth filtered signal; and the second negative path comprises a second linking circuit connecting the second high-pass filter and the first low-pass filter and configured to match gains of the second filtered signal and the third filtered signal.
 14. A method for communicating high speed signals between an input module and an output module, the method comprising: receiving a first differential signal from [[an]] the input module, the first differential signal having a first predetermined common mode voltage and including a first positive differential signal and a first negative differential signal; providing the first positive differential signal to a first high-pass filter and a first low-pass filter; filtering the first positive differential signal with the first high-pass filter and the first low-pass filter to obtain a first filtered signal and a second filtered signal respectively; adjusting the first predetermined common mode voltage in the first positive differential signal with the first low-pass filter to obtain a second predetermined common mode voltage in the second filtered signal; providing the first negative differential signal to a second high-pass filter and a second low-pass filter; filtering the first negative differential signal with the second high-pass filter and the second low-pass filter to obtain a third filtered signal and a fourth filtered signal respectively; adjusting the first predetermined common mode voltage in the first negative differential signal with the second low-pass filter to obtain a third predetermined common mode voltage in the fourth filtered signal; cross-coupling an output of the first high-pass filter with an output of the second low-pass filter such that the first filtered signal is combined with the fourth filtered signal to form a second positive differential signal; cross-coupling an output of the second high-pass filter with an output of the first low-pass filter such that the second filtered signal is combined with the third filtered signal to form a second negative differential signal; providing the second positive differential signal and the second negative differential signal to the output module, wherein the second positive differential signal and the second negative differential signal form a second differential signal having a fourth predetermined common mode voltage less than the first predetermined common mode voltage, the fourth predetermined common mode voltage corresponding to an acceptable input voltage level for the output module.
 15. The method of claim 14, further comprising: inverting the first positive differential signal with the first low-pass filter such that the second filtered signal is inverted with respect to the first positive differential signal; and inverting the first negative differential signal with the second low-pass filter such that the fourth filtered signal is inverted with respect to the first negative differential signal.
 16. The method of claim 14, further comprising: comparing a sum of the second predetermined common mode voltage and the third predetermined common mode voltage to a predetermined reference voltage; and generating a voltage input for the first low-pass filter and the second low-pass filter in response to the comparing, the generated voltage input controlling the second predetermined common mode voltage in the second filtered signal and the third predetermined common mode voltage in the fourth filtered signal.
 17. A communication device comprising: a transceiver configured to provide a first differential signal carrying a data stream at an output connection, the first differential signal comprising a first positive differential signal and a first negative differential signal and having a first common mode voltage at the output connection; a processing module comprising an input connection to receive a second differential signal, the second differential signal comprising a second positive differential signal and a second negative differential signal and having a second common mode voltage, the second common mode voltage being different from the first common mode voltage; and a coupling module connecting the transceiver and the processing module, the coupling module comprising: a positive path configured to receive the first positive differential signal from the output connection of the transceiver and provide the second positive differential signal to the input connection of the processing module, the positive path comprising a first high-pass filter configured to provide a first filtered signal connected in parallel with a first low-pass filter configured to provide a second filtered signal, wherein the first filtered signal and the second filtered signal are each based on the first positive differential signal; a negative path configured to receive the first negative differential signal from the output connection of the transceiver and provide the second negative differential signal to the input connection of the processing module, the negative path comprising a second high-pass filter configured to provide a third filtered signal connected in parallel with a second low-pass filter configured to provide a fourth filtered signal, wherein the third filtered signal and the fourth filtered signal are each based on the first negative differential signal, wherein the second positive differential signal is formed from a combination of the first filtered signal and the fourth filtered signal and the second negative differential signal is formed from a combination of the second filtered signal and the third filtered signal; and a feedback circuit coupled to the first low-pass filter and the second low-pass filter, the feedback circuit configured to control the second common mode voltage in the second differential signal independent of the first common mode voltage in the first differential signal.
 18. The communication device of claim 17, wherein the feedback circuit is configured to receive the second filtered signal and the fourth filtered signal and provide an input to the first low-pass filter and the second low-pass filter such that the first low-pass filter adjusts a common mode voltage level in the second filtered signal and the second low-pass filter adjusts a common mode voltage level in the fourth filtered signal such that the common mode voltage level of the second filtered signal and the common mode voltage level of the fourth filtered signal results in the second common mode voltage in the second differential signal.
 19. The communication device of claim 18, wherein the feedback circuit comprises an operational amplifier having a non-inverting input configured to receive a voltage and an inverting input configured to receive a sum of the common mode voltage levels for the second filtered signal and the fourth filtered signal.
 20. The communication device of claim 19, wherein the feedback circuit further comprises a stabilization circuit connected between the output of the operational amplifier and the inputs of the first low-pass filter and the second low-pass filter, the stabilization circuit comprises a resistor connected in parallel with a capacitor.
 21. The communication device of claim 17, wherein: the first high-pass filter and the second high-pass filter each comprise a capacitor; and the first low-pass filter comprises a first operational amplifier, the first operational amplifier configured to operate in an inverting mode such that the second filtered signal is inverted with respect to the first positive differential signal; and the second low-pass filter comprises a second operational amplifier, the second operational amplifier configured to operate in an inverting mode such that the fourth filtered signal is inverted with respect to the first negative differential signal.
 22. The communication device of claim 17, wherein: the positive path further comprises a first linking circuit connecting the first high-pass filter and the second low-pass filter and configured to match gains of the first filtered signal and the fourth filtered signal; and the negative path further comprises a second linking circuit connecting the second high-pass filter and the first low-pass filter and configured to match gains of the second filtered signal and the third filtered signal.
 23. The communication device of claim 17, further comprising: an adapter module connected between the coupling module and the transceiver, the adapter module having circuitry configured to receive the first positive differential signal and the first negative differential signal from the transceiver; the positive path includes a first positive path portion coupled to the first high-pass filter and a second positive path portion coupled to the first low-pass filter; the negative path includes a first negative path portion coupled to the second high-pass filter and a second negative path portion coupled to the second low-pass filter; and wherein the circuitry of the adapter module is configured to: provide the first positive differential signal to the first positive path portion; provide the first negative differential signal to the first negative path portion; modify the first positive differential signal and the first negative differential signal to generate a first modified positive differential signal and a first modified negative differential signal; provide the first modified positive differential signal to the second positive path portion; and provide the first modified negative differential signal to the second negative path portion.
 24. The communication device of claim 23, wherein the circuitry of the adapter module comprises a resistor divider connected to a voltage adjustment circuit, the resistor divider is configured to provide the first modified positive differential signal and the first modified negative differential signal to the second positive path portion and the second negative path portion of the coupling module, and the voltage adjustment circuit is configured to provide a predetermined voltage level to the resistor divider. 